Display devices included in mobile phones such as smartphones, tablet information terminals, and notebook personal computers (PC) have undergone various improvements in recent years. For example, there have been developed display devices with features such as higher resolution, higher color reproducibility (higher NTSC ratio), a smaller driver circuit, and lower power consumption.
As an example, an improved display device has a function of automatically adjusting the brightness of an image displayed on the display device in accordance with ambient light. An example of such a display device is a display device having a function of displaying an image by reflecting ambient light and a function of displaying an image by making a light-emitting element emit light. This structure enables the brightness of an image displayed on a display device to be adjusted in the following manner: the display device is set to a display mode for displaying an image with use of reflected light (hereinafter referred to as a reflective mode) when ambient light is sufficiently strong, whereas the display device is set to a display mode for displaying an image with light emitted from a light-emitting element (hereinafter referred to as a self-luminous mode) when ambient light is weak. In other words, the display device can display images in a display mode that is selected from the reflective mode, the self-luminous mode, and a mode using both the reflective and self-luminous modes in accordance with the intensity of ambient light sensed with an illuminometer (illuminance sensor).
As examples of a display device having a function of displaying an image by making a light-emitting element emit light and a function of displaying an image by reflecting ambient light, Patent Documents 1 to 3 each disclose a display device in which one pixel includes a pixel circuit for controlling a liquid crystal element and a pixel circuit for controlling a light-emitting element (such a display device is referred to as a hybrid display device).
For image processing for a display device to display an image, the utilization of a neural network has been considered. Furthermore, Non-Patent Document 1 discloses a technique relating to a chip having a self-learning function with the neural network.